It is often necessary or desirable to measure the cardiac output of a patient. A common technique for accomplishing this is to inject a known volume of an injectate, such as a saline solution, into the right atrium. The injectate has a known temperature and it mixes with the blood to produce a temperature drop in the blood. The temperature of the blood is monitored at a suitable location downstream of the right atrium, and the data obtained can be used to determine cardiac output. This technique is commonly referred to as thermodilution, and the injection of the injectate and the downstream temperature measurement are carried out by a thermodilution catheter.
It is know to obtain the upstream temperature measurement of the injectate using a thermistor permanently mounted in the conduit which supplies the injectate. Although this system functions satisfactorily, it is necessary that the thermistor be initially sterile and be discarded after each usage. This increases the cost of using the thermodilution technique because the thermistor is a relatively expensive component. In addition, the thermistor bead material is subject to being attacked by the injectate.
The injectate may be at room temperature or at reduced temperatures. In the reduced temperature system, the injectate is cooled at one location and manually transported to the injection location near the proximal end of the thermodilution catheter. With this system, there is a risk of loss of sterility, the injectate may not be as cold as desired and there is an absence of a constant, ready supply of the injectate.